1. Field of the Invention
The present invention relates to a method of producing low-resistant monocrystalline metal layers on substrates, such as are utilized in semiconductor and thin film technologies.
2. Prior Art
Metal or alloy layers (i.e., metallic layers) on substrates composed of ceramic, glass or silicon are frequently utilized in electro-technology. Such relatively thin metallic layers, for example, function as lead or conductor structures, electrical resistance or capacitors in semiconductor or thin film technologies.
Continuing miniaturization in semiconductor and thin film technology is accompanied by a reduction of the cross-section of lead or conductor structures. Accordingly, the height to width ratio of the lead structure becomes significantly greater as the heat dissipation across the substrate becomes poorer, i.e., the lead structure is subjected to a higher thermal load. Because the current strength cannot be reduced proportionally to the lead cross-section, it must be able to withstand current densities of 1.times.10.sup.6 through 1.times.10.sup.8 A/cm.sup.2. Both higher thermal loads as well as higher current densities accelerate the failure of leads due to electro-migration. A primary cause of electro-migration appears to result from material inhomogeneities, for example, at crystal boundaries.
As utilized herein and in the claims, the term "monocrystalline" is utilized to designate structures wherein the mean grain diameter, k, is significantly greater than the layer thickness, d, (typically, k.gtoreq.100.times.d), but at least achieves the size of, for example, a transistor in a system (with k being approximately equal to 50 .mu.m).
Heretofore, problems of electromigrations were solved by the use of low-resistant materials (.rho.&lt;15 .mu..OMEGA. cm) and current densities up to a maximum of about 5.times.10.sup.5 A/cm.sup.2. Since the effect of electromigration is greatest when k (mean grain diameter) approximately corresponds to the width of the leads, two possibilities were available:
1. Production of fine-crystalline materials. A stable, amorphous material would be ideal, but this would hardly be useful because of the high specific electrical resistance which necessarily follows. For this reason, a compromise was undertaken in which, for example, aluminum was alloyed with 1.2 through 2% silicon and/or a maximum of 4% copper. This relatively small number of foreign atoms increased the specific electrical resistance only slightly; in comparison to pure aluminum, the foreign atoms stabilized the structure of the lead material so that, in the final analysis, an improvement of the long-term behaviour resulted (for further details see D. R. Denison et al, Technical Report, No. 79.02, Perkin Elmer, Ultek Division, pages 1-8, April, 1979.) PA1 2. Production of monocrystalline layers. Investigations by d'Heurle et al (Applied Physics Letters, Vol. 16, pages 80-81, January 1970) have shown that monocrystalline aluminum would meet all future demands in terms of its long-term stability and resistance to electromigration. However, a problem exists in that the production of such monocrystalline aluminum layers has heretofore only been possible on monocrystalline substrates at temperatures above 350.degree. C.
A method by which monocrystal layers can be obtained without temperatures above 350.degree. C. being necessary, is known from German Offenlegungsschrift 27 27 659. In accordance with this technique, stable layer properties are achieved by depositing metal layers, preferably composed of tantalum, on a substrate in an amorphous or greatly disrupted state at a low substrate temperature, for example, at -190.degree. C. and, subsequently comparatively slightly heating the substrate to cause the deposited metal layer to crystallize. In this manner, coherent lattice areas of less than 4 nm are enlarged into crystals having a grain diameter of at least 70 .mu.m by heating the substrate above about -90.degree. C. Such layers retain their relevant properties even at much higher temperatures (higher than 400.degree. C.), although they were not exposed to any temperature higher than -90.degree. C. during their production process. This method can be utilized in semiconductor and thin film technologies but, up to now, has only yielded layers with k.apprxeq.100.times.d, with tantalum, tantalum-cobalt and tungsten-copper. After crystallization, the specific electrical resistance of these layers is still above 15 .mu..OMEGA. cm.